In this appendix,
I've included an exchange with a modeler that was concerned about how to
retrofit IR sensors into completed modules.The response was provided by Gregg Fuhriman with some further
comments of mine.I'm
confident that you will find the installation hints helpful.

Before getting to that, for those that may need
more information about the Modular Signaling System (MSS), I have
included the following:

Comprehensive documentation of the
V2.2.0 MSS
Standard is now available. Information is included for both
developers and users.

The
OBUB-3,
designed by Gregg Fuhriman, makes MSS installation much easier and more compact.

In case you wonder
how signals should operate, there are several resources that you might find
helpful, especially if you're an ATSF modeler.One
is a detailed summary
of the most likely operation of signals on the single track second district (the
"Pasadena
district") in Southern California.This page was created for me by Carsten Lundsten with help from
Barry Draper.Carsten has
additional information on signal operation for some other railroads
that you may find helpful.

I prepared a summary of
Carsten's information for
implementation on the Shandin loop module.Both Carsten and Barry Draper were helpful in making sure that I
got it right.There were
some of the conditions that I couldn't implement in the absence of the
ability to sense the direction of trains.

The Concern: Retrofitting IR Sensors

In talking with others in our group, the issue, and potentially the

difficulty, of
installing optical detectors in completed modules has become a sticking
point. Can you share your experience of installing these in Glen Frazier
and other completed modules? Any tips or suggestions?

Gregg Fuhriman's Reply

I too had
reservations about this until I just dove in and did it ... turns out
its not all that difficult. Here is how I did it on Glen Frazer, to
mount the Optek OPB704W sensor. This was going through cork roadbed and
blue foam. Going through a plywood module top is a different story;
perhaps Gary Green (cc'ed) can explain how he that.

Before installing
the
Optek
sensor, you may want to mount a connector of some sort onto its 4
wires, for modularity and ease of replacement should it ever fail. Also,
the wires are sort of short, so extending them is likely required.

Also before
installing, it is possible to
bevel the
sides of the sensor housing down away from the window, narrowing the
visible portion of the sensor to more closely match a track tie width.
Do not remove material right up to the window though! Leave a thin
portion of the housing along the edge of the window to block stray light
from leaking in.

1.Buy a set of the #11-sized saw blades from Micro-Mark.I think I used the finest-toothed blade in a standard Xacto knife
handle.

2.Choose the tie where the sensor is going to go (make sure there
is clearance under it, e.g. no frame member in the way, etc.), and
carefully cut out the central section using the saw blade. This is the
most difficult part and requires a fine touch and a bit of patience. The
opening needs to be slightly wider than the tie, and the tie ends must
be angled (undercut) to match the profile of the sensor. You may lose a
bit of ballast, but it will be patched up later.

3.Form the hole through the module top (working from above and
below as needed) to fit the sensor profile and size. This may require
hand saws, files, knives, whatever. It's OK if the hole ends up slightly
oversized, but make it as small as possible, especially at track level.

4.Test fit the sensor by pushing it up through the hole from below.
The goal is to have it fit snugly, square, flat, and centered in the
selected tie, with the sensor surface flush with the top of the tie.
Keep test fitting and adjusting the hole until the goal is met.

5.When you have a good fit, install the sensor in its final
position, and shoot some expanding insulation foam from below, into the
gaps around the sensor. Try not to use too much so the foam does not
leak up onto the top of the sensor or surrounding track. Hopefully the
hole profile at track level is tight enough to prevent this. You have
time to make any final positioning adjustments as the foam sets up. This
type foam can be removed fairly easily, should the sensor need to be
replaced for some reason. I do not recommend using glue (latex liquid
nails, etc.) as it becomes quite hard and therefore very difficult to
remove cleanly.

6.Once the foam is dry, use a small piece of masking tape to cover
the sensor window. Carefully repair the ballast next to the sensor; if
you have bevel it, the ballast will come right up against the window. Of
course make sure no ballast is on top the sensor window. Once the
ballast is dry, if you desire, weather the track so everything matches
again. Remove the tape from the window once all the messy work is done.

That's it ... now
just connect the sensor to the optical detection circuit.

MY ADDITIONAL COMMENTS:

I'll add some
comments to Gregg's just in case there's another plywood nut there:

Most
important...whatever work it is to install the IR detectors is worth the
trouble! It's very disappointing to install detection and then to have a
block go clear when there's still a long train in the block and only the
engines have exited the block. And there's nothing more rewarding than
to have the current and IR detection working in concert to provide block
occupancy that "sees" the whole train. As much as I complained while
installing the detection, I would do it again in a heartbeat.
Install IR detectors at present
or future block boundaries without delay. I'm confident that you won't
be disappointed.

My sensors were
retrofitted into a "completed" module (track in, ballasted, in operation
for some years).

I followed Gregg's
recommendation to install a connector on the sensor wires and I made
certain that I provided for a good service loop in the wires leading to
the Heathcote PCB. The connector allowed me to install the sensor and
the PCB more easily and also gave me the ability to disconnect the
sensor for debugging and testing.

I beveled the
sensor as Gregg recommends. It ends up only slightly wider than a
Walther's code 83 tie. My bevel ends in almost but not quite a knife
edge adjacent to the IR window. I masked the IR window at this point.
Thinking that the remaining housing next to the window might not be
fully opaque, I painted the housing surface on the bevel with black
paint just in case it might help.

On my plywood top,
after cutting away the center of the tie where the sensor was to go, I
"marked" the location by drilling a small hole through the
subroadbed (module top) so I could identify the location from
underneath. The small hole
served as a locator for the Forstner drill used in the next step.

I purchased a
Forstner drill bit large enough to accommodate the full width of the
sensor, marked the proper drilling depth on it with some high adhesion
masking tape, and carefully drilled out most of the thickness of the plywood so
that only a relatively thin layer of the plywood remained (I tried for
1/8") under the cork roadbed. This was a scary process, but it worked
out OK. Then
from the top, I cut the cork out with a knife and proceeded to remove
the thin layer of plywood as required to allow me to push the sensor
into position. Because I was working with plywood subroadbed, I had to
work harder than Gregg to open up the slot, and I used more aggressive
tools. It took some real thought and care not to damage the rail and
adjacent ties. As you can imagine, the slot wasn't pretty, but you can
only tell that from underneath now.

Since I was
working with plywood and could knock the sensor out if it failed, I
glued the sensor in place using epoxy glue. The glue helped to fill the
spaces between the sensor body and the slot so that ballast didn't leak
through when it was added around the sensor.I managed to cut
the ties a bit wider than needed, so I filled the gaps between the
sensor body and the tie "ends" with a putty like Squadron Green (I use
an auto body putty that works well), shaped it as best I could and
painted it so that it looked like I had cut the ties to exactly the
correct length.Re-ballasting
wasn't any harder than it always it.

I made the mistake
of placing the sensors *exactly* at the location of the rail gaps
separating the blocks. When I cut the tie to accommodate the sensor, I
lost the structural integrity of the tie strip needed to hold the rail
in place adjacent to the gap. This matters on Shandin because all the
sensors are located on curves and the rail seems never to lose it's
desire to straighten out. I had to do a lot of jimmying and ACCing to
successfully stabilize things. In the future, I'll offset the sensor
several ties one way or the other from the rail gaps, even on straight
track.

The LED on the
IRDOT is very useful. Don't fail to keep it in place.
I tuned the
resistor on the PCB so that it's time delay (how long before it turns
off when nothing is detected) matches the time delay of the current
detectors I use (DCCODs).